Genetic recombination in mammalian cells is an essential component in growth and differentiation. It is also the mechanism underlying chromosomal rearrangements characteristic of supergene families (such as immunoglobin genes) and is responsible for many karyotypic abnormalities associated with some forms of cancer. Little is known, however, concerning the mechanisms of, and cis- and trans-acting elements involved in, the process of recombination in mammalian cells. In the proposed study, herpes simplex virus type 1 (HSV-1) system will be used as both a tool and model system to study recombination in mammalian cells. A system will be developed in which recombination between viral DNA sequences inserted into the mammalian chromosome and homologous sequences in HSV-1 genomes can be observed and quantified. The effect on recombination frequency of various HSV-1 cis-acting elements linked to cell-associated substrate will be determined. The cis- acting elements to be studied include an HSV-1 origin of viral DNA synthesis, the "a" sequence, and a reiterated element which others have shown to be homologous to sequences found in immunoglobin class switch regions. The effect of HSV-1 trans- acting elements on recombination will also be explored. For this purpose, a system able to detect and quantify homologous recombination in the genetic backgrounds of a series of well- characterized temperature-sensitive mutants will be developed. Through the use of a rapid enzymatic assay, cloned HSV-1 fragments encoding gene(s) whose products are involved in site- specific recombination mediated by reiterations such as the "a" sequence will be identified. To complement and extend these studies, mutants defective (or altered) in site-specific and general recombination will be isolated and characterized. Taken together, these studies should shed light on the cis- and trans-acting viral elements involved in recombination thus providing a basis for transformation and transduction systems for mammalian cells with specific eukaryotic genes and DNA sequences.